Hard disk drives (HDD) are provided with a magnetic recording disk and a magnetic head, and data on the magnetic recording disk is read and written by means of the magnetic head. A magnetic head in an HDD comprises a write head for recording data onto the magnetic recording disk as magnetization signals, and a read head for reading the signals which are recorded as magnetization signals on the magnetic recording disk. The read head comprises a magnetoresistive layered structure formed from a plurality of magnetic thin films and non-magnetic thin films. The read head reads the signals using the magnetoresistive effect and therefore it is referred to as a “magnetoresistive head.”
There are several kinds of layer structures for magnetoresistive heads, and these are divided into anisotropic magnetoresistance (AMR) heads, giant magnetoresistance (GMR) heads, current perpendicular-to-plane GMR (CPP-GMR) heads, tunneling magnetoresistance (TMR) heads, and the like, according to the magnetoresistive principle used therein (see Japanese Unexamined Patent Application Publication H11-191207, for example). Input magnetic field signals which enter the read head from a magnetic recording medium are retrieved as voltage changes using AMR, GMR, CPP-GMR, and TMR, respectively.
Developments in increased density of recorded data have meant that there is currently a need for more sensitive systems for reproducing data signals. When the recording density is 500 Gb/in2-2 Tb/in2, TMR read heads which have a very high magnetoresistance (MR) ratio are useful because of greater sensitivity. TMR read heads employ a tunneling effect in a layered structure comprising an insulating layer and two ferromagnetic layers, one either side. One of the two ferromagnetic layers is a fixed layer in which the magnetization direction is fixed, while the other is a free layer in which the magnetization direction is changed by the external magnetic field.
TMR causes differences in the density of electrons having a different spin orientation to the orientation of the magnetization of the magnetic material and electrons having the same orientation. The resistance value of a TMR layered structure varies according to the relative angle between the magnetization direction of the fixed layer and the magnetization direction of the free layer. When the magnetization direction of the fixed layer and the magnetization direction of the free layer are parallel, the resistance is at its lowest, but when the magnetization direction of the fixed layer and the magnetization direction of the free layer are antiparallel, the resistance is at its highest. The magnetization direction of the free layer changes according to the magnetic field from the magnetization on the magnetic recording disk, and this causes the resistance value of the TMR layered structure to change. The HDD acquires the magnetization information on the magnetic recording disk by detecting this resistance value.
In one approach, it is possible to reduce the track width by increasing the recording density in the magnetic recording disk, and to reduce the size of the magnetic head accordingly. A highly sensitive TMR layered structure which may deal with a high recording density is needed in order to obtain the required reproduction output for the read head. Annealing the magnetic layers of the TMR layered structure at high temperature is effective for achieving a high MR ratio. Crystallization of the magnetic layers is promoted by the annealing treatment, and scattering of the electron spin is suppressed, whereby a high MR ratio is achieved. Magnetic heads are therefore currently produced by stacking the layers of the TMR layered structure and then carrying out high-temperature annealing.
Another effective method for increasing the MR ratio is to produce a fixed layer having high magnetization strength in one direction. It is possible to achieve large variations in magnetic resistance by means of a fixed layer having high magnetization strength in one direction and a free layer wherein the magnetization orientation varies according to the external magnetic field. The fixed layer comprises a first ferromagnetic layer, a second ferromagnetic layer, and an antiparallel coupling layer therebetween. The first ferromagnetic layer and second ferromagnetic layer generally include Co and Fe, and comprise a single layer or multiple layers. Furthermore, the antiparallel coupling layer comprises Ru. It is necessary to increase the antiparallel coupling strength between the first and second ferromagnetic layers in order to improve the strength and stability of the fixed layer.
In order to strengthen the antiparallel coupling between the first and second ferromagnetic layers, in other words to increase the antiparallel exchange coupling constant JRu (erg/cm2), it is effective to thin the antiparallel coupling layer. As described above, in order to promote crystallization of the magnetic layers in the TMR layered structure and increase the MR ratio, it is effective to increase the annealing temperature.
However, research carried out by the inventors revealed a problem in that the MR ratio deteriorates if the annealing temperature is raised when the antiparallel coupling layer is thin. This is believed to be due to the fact that high-temperature annealing causes increased dispersal of the ferromagnetic layers of the fixed layer in the magnetization direction, making it difficult to produce parallelism/antiparallelism. It is believed that dispersal in the magnetization direction weakens the antiparallel coupling of the first and second ferromagnetic layers, and the fixed layer shows unstable characteristics.
There is therefore a need for technology for TMR read heads which makes it possible to increase the MR ratio by promoting crystallization of the magnetic layers through high-temperature annealing, and which makes it possible to increase the coupling strength of the fixed layer. By achieving these two aims, it is possible to achieve a higher MR ratio in a TMR read head.